The present invention relates generally to the field of electronic circuits and more particularly to a photodetector preamplifier circuit having a xe2x80x9crotatingxe2x80x9d input stage.
There are a number optical storage standards such as CD (compact disks), DVD (digital video disks), CD-RW (Write/Read CDs), etc. All of these products require photodetector preamplifiers to sense and amplify the reflection from the disks.
There is a trend to build a single machine that can read all of the different standards. Each of these different standards have different amounts of reflectance of the interrogating laser. Thus, a preamplifier circuit designed for one standard is not optimum for a second standard. This can result in misread bits and degrade the performance of the optical storage system. Generally, a preamplifier with adjustable gain is desirable for best performance.
However, the photodetector/preamplifier circuit has a number of other important constraints that make adjustable gain more difficult. The main other constraints include: low-noise amplification, wide signal bandwidth, DC accuracy, relatively large photodetector size, high responsivity, and low cost. These other constraints increase the challenge of providing adjustable gain.
This problem can be demonstrated by an example. A typical photodetector preamplifier circuit is shown in FIG. 1. The photodetector is represented by the current source 14 and the capacitor Cpd, 16. The preamplifier, 10, is represented in its simplest form by the amplifier symbol and the feedback resistor Rf. The gain of the preamplifier is approximately set by the resistor Rf; which converts the input photocurrent to an output voltage, where this conversion factor is called the transimpedance gain. The bandwidth is set by the amplifier characteristics and by the capacitance of the photodiode (plus other parasitic capacitances). The amplifier, plus the resistor RF and the photodetector capacitance Cpd form a feedback loop. This loop is potentially unstable, and the standard stability criteria and calculations (Nyquist, Bode, etc) must be used to guarantee stability over all process, temperature and other production variations. The dominant pole in the loops is usually created by the time constant of the feedback resistor and the photodetector capacitance. In order to achieve the low noise objective, the feedback resistor must be large. In order to achieve wide bandwidth, the amplifier gain must be large enough to move the dominant pole to a high frequency in the closed loop. And, to achieve DC accuracy, the amplifier gain must be quite high at low frequency.
Normally, to achieve DC accuracy and high gain, an amplifier will be constructed in the standard industry practice of an input differential stage, followed by a high gain 2nd stage and then frequency-compensated by xe2x80x9cMillerxe2x80x9d a feedback capacitor across the 2nd stage. This produces a standard operational amplifier with DC accuracy, high gain, and possibly wide closed-loop bandwidth. However, using this in a photodetector preamplifier will lead to either reduced bandwidth or to instability. The dominant pole of the operational amplifier, combined with the dominant pole of the Rf, Cpd will result in a 2nd order loop. If additional poles and time delays are introduced (as a result of parasitics or other portions of the amplifier), then the result is an oscillator.
The normal methods to reduce this oscillation are: 1) reduce the bandwidth of the amplifier so that it is the dominant pole of the system; 2) reducing the size of Rf to increase the frequency of the Rf Cpd pole; 3) compensate Rf by placing a capacitor in parallel with it; and/or 4) compensate Cpd by placing a resistor in series with it. These solutions all result in sub-optimal preamplifiers; 1 and 2 result in much lower bandwidth, while 3 and 4 result in much higher noise levels.
These problems are made much more difficult when the feedback resistor, Rf needs to be variable. Then, any of these compensation methods is more difficult due to the multiple criteria that must be simultaneously optimized.
Another practical difficulty in implementing very wide bandwidth preamplifiers is the trade-off between DC accuracy and bandwidth. This trade-off occurs in 2 main ways: 1) in the location of the dominant poles of the amplifiers; and 2) in the physical size of the components of the amplifier. The first part of the trade-off has been described above, but the 2nd requires a bit more explanation. In order to created extremely wide bandwidth amplifiers, any parasitic capacitances must be reduced to a minimum. These parasitic capacitances are simply the result of physical dimensions of components, so it is important to use very small transistors throughout the amplifier. However, it is a well-established fact that mis-match between transistors is reduced if the transistors are made much larger than the minimum allowed by the process technology in use. If xe2x80x9cminimum sizexe2x80x9d transistors are used, then the mis-match is large.
The input stage of an amplifier generally consists of a differential pair to measure the input differential voltage, and amplify it. If the transistors are mis-matched, then a 0 differential voltage could appear as if there was a signal present; or an input signal can be partially or fully cancelled by the mis-match of the transistors.
Thus, it is difficult to simultaneously optimize the DC accuracy of an amplifier and also achieve maximum bandwidth. This problem is made worse due to the stability constraints of a photodetector preamplifier, and it is difficult to meet all the criteria of performance in a low-cost manufacturing process.
It is the object of this present invention to provide: a) an amplifier topology that allows for optimum dynamic range, bandwidth, transimpedance gain, signal to noise ratio, and DC accuracy; b) a means of allowing the transimpedance gain to be varied while the other criteria are optimized, and c) an improved DC accuracy for the input stage of this amplifier.